ATMOSPHERIC PARTICLES AND ELECTRICAL CONDUCTIVITY DURING DIFFERENT WEATHER CONDITIONS

Abstract

Atmospheric aerosols or suspended particulates are the dispersed solid or liquid or mixture of both kind of particles in air which are most complex and least understood atmospheric constituents. These particles are present in the atmosphere with varied chemical composition and wide range of size from 0.001 fam to 100 \xm in radius. Study of such atmospheric particles is important to understanding their influence directly or indirectly on climate and human health. The atmospheric electrical parameters, viz. atmospheric electrical conductivity, air-earth current density, electric field etc. are affected by various environmental and meteorological factors. The atmospheric particles and electrical parameters can not be in a steady state at all the times at a particular place. Therefore a clear knowledge of the nature of atmospheric particle concentration and their distribution and atmospheric electrical conductivity at a given location during different weather conditions (disturbed and fair-weather) is very important to characterize the aerosols and particulate system over the location and also to get information on the meteorological condition in lower atmosphere. The region lying between 65-70 km to about 1000 km altitude above the earth surface containing free electrons and ions is known as ionosphere and is primarily responsible for introducing errors in transionospheric radio signals. The equatorial ionosphere is formed primarily by the ionization of neutral gas atoms/molecules present in the upper atmosphere by exposure to solar radiation. The free thermal electrons, having energies of a few tens of electron volts and produced by high energy photoelectron collision (i) with neutrals, form more than 90% of the flux in the ionosphere. The weather disturbances like thunderstorms may alter the ion-electron production rate and hence change the ionospheric electrical parameters. The characteristics of these ion-electrons are studied in order to characterize the ionosphere. The present thesis presents some experimental studies on atmospheric particle concentration, their size distribution and atmospheric electrical conductivity in the light of some meteorological parameters (like temperature, humidity, rain, wind speed etc.). It also includes a study on the low latitude ionosphere and the behavior of electrons and ion density and temperature anomaly in the low latitude region. The data for this purpose has been obtained from Retarding Potential Analyzer (RPA) payload of SROSS-C2 mission, operative since May 1994. This is a combined effort of Indian Space Research Organization (ISRO), National Physical Laboratory (NPL) and the University of Roorkee (UOR). A brief survey on the present-state-of-art regarding aerosols and particulate generation and distribution has been presented in Chapter-I. The different mechanisms involved in aerosol formation and their distribution in the atmosphere in different weather conditions has been described in this chapter and has the potential to advance knowledge in this area. It also includes various studies on atmospheric electrical parameters with respect to the environmental effects including the particulate pollution. It also includes the existing knowledge about the ionospheric phenomenon. Since the characteristics of the ionosphere show a large spatial and temporal variation, its long term study helps in improving the existing reference ionospheric models being used to apply ionospheric corrections. In brief, this chapter gives introductory remarks to the existing knowledge and the glimpse of the work carried out for the present study in subsequent chapters. Chapter-ll deals with the instrumentation part of the thesis. It gives brief description of the instruments fabricated for present studies. The atmospheric particle concentration and their size distribution are measured by using a specially designed laser scatterometer. It works on the principal of Mie Scattering with an assumption that the atmospheric particles follow the normal Gaussian distribution in the atmosphere. This scetterometer involves 5mW He- Ne laser, photo-diode and two strip chart recorders. For measuring the atmospheric electrical conductivity a Gerdian Condenser was fabricated. It uses a sensitive electrometer amplifier, high voltage power supply, multichannel chart recorder and blower having a sucking capacity of 1500 liter/minute. Gerdian condenser is a cylindrical capacitor consisting of a central electrode (conducting rod) and an outer electrode (cylindrical tube). The function of various parts, working principle and theoretical concepts of Gerdian Condenser have been described in detail. The tropospheric aerosols and electrical environment are closely related to the solar activity. Atmospheric ions play an important role in governing the aerosols and electrical conductivity at ground surface. Solar Cosmic Rays (SCR) and Galactic Cosmic Rays (GCR) are the chief sources of ionization in the atmosphere. During solar eclipse the moon stops GCR and SCR particles and the effect is felt to the ground surface. Therefore an experimental study on aerosol concentration and size distribution as well as atmospheric electrical conductivity has been performed during the total solar eclipse of October 24, 1995, and is reported in Chapter-Ill. The variance of these factors in view of some meteorological parameters has also been presented. The result shows that both aerosol concentration and electrical conductivity increases during the solar eclipse; which is contrary to the normal atmospheric conditions. It has been argued that the causes are not local. The stoppage of solar wind flux by moon increases the GCR at lower altitudes thereby causing the increased ionization. The increased ionization during the solar eclipse is responsible for the increase of atmospheric electrical conductivity. Also the atmospheric ions increase the cloud condensation nuclei which plays an important role in aerosol formation and hence the increase in aerosol concentration. Various meteorological changes may have influence on the atmospheric aerosols and the surface electrical conductivity. Therefore it may be very helpful to study the nature of aerosols and electrical conductivity during different weather conditions. For this purpose we have chosen the monsoon period and winter season as disturbed and fair-weather atmospheric conditions respectively. The experimental studies on tropospheric aerosols and electrical conductivity have been done for the monsoon period (June-September, 1996) and the fair-weather region (November, 1996-February, 1997) at Roorkee, India. The results of these studies have been presented in Chapter-IV. The atmospheric electrical conductivity has been found to decrease with increasing relative humidity and temperature. Wind was an important factor, which modified the behavior of conductivity in short-term range. The atmospheric aerosol concentration and size were found to decrease with increase in rainfall. Also, the relative humidity plays an important role and changes the aerosol density distribution. However, relative humidity less than 95% do not affect effectively the aerosol concentration. The ionospheric weather has become the most important topic of the day. It is affected by the solar geomagnetic disturbances and the tropospheric fluctuations. As a result of extensive research work, it is now well known that the ionospheric system varies markedly with altitude, longitude, universal time, season, solar cycle, geomagnetic activity and solar flares. The ionospheric parameters can vary appreciably from hour to hour or from day to day and it can display a considerable amount of structure. The ion and electron density fluctuations are associated with the ionospheric structure, where thermalized electrons play very important role. Even the small fluctuations in ion-electron density can cause signal fluctuations, which affect HF communication. Therefore Chapter-V is dedicated to the studies of ion-electron structure, with an objective to study the behavior of electron and ion number densities and temperature anomaly in the low latitude region. The data for this purpose has been obtained from RPA payload aboard SROSS-C2 satellite launched by ISRO. This chapter also includes the RPA payload and spacecraft configuration for measuring ion-electron densities and temperature along with the RPA data reduction and analysis. The present study reveals the fact that the behavior of atmospheric aerosols distribution and electrical conductivity are very much affected by some processes like solar eclipse and disturbed and fair weather conditions. The major meteorological factors like wind speed, temperature, rain fall and relative

humidity plays an important role in characterization of the tropospheric aerosols and atmospheric electrical conductivity. Studies during the solar eclipse show that SCR and GCR also play an important role in the atmospheric processes. The investigations regarding ionospheric ion and electron densities and their temperature anomalies reveal the fact that these parameters are highly affected by different weather conditions in the troposphere. The summary and conclusions regarding the present work and suggestions for future studies have been given in Chapter-VI.